1. Field of the Invention
This invention relates generally to telecommunication, and, more particularly, to a wireless telecommunication system.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more networks. Mobile units, such as mobile telephones, personal data assistants, Global Positioning System devices, desktop or laptop computers, and the like, located in each cell may access the wireless communications system by establishing a wireless communication link with a base station associated with the cell. For example, a mobile telephone may initiate communication with a base station by providing a signal on an access channel. The base station may then use the received access channel signal to establish the wireless communication link between the mobile unit and the base station on a separate traffic channel.
At any given time, each base station may be expected to maintain concurrent wireless communication links with numerous mobile units. To reduce interference between the concurrent wireless communication links, the base station and the mobile units in the wireless communication system modulate signals transmitted on the assigned traffic channels using a predetermined code that uniquely identifies the mobile unit. For example, in a wireless communication system operating according to the CDMA 2000 standard, a public long code mask may be used to differentiate reverse link transmissions, i.e. from the mobile unit to the base station, over different traffic channels. The public long code mask is typically a 42-bit-long mask including two bits that indicate the type of the long code mask (public or private), an additional eight bits that provide signaling information such as the method used to form the long code mask, and a 32-bit electronic serial number (often referred to using the acronym ESN) assigned to the mobile unit.
The proliferation of various types of mobile units has begun to exhaust the supply of 32-bit electronic serial numbers. To accommodate the growing number of mobile units, many practitioners have proposed replacing the 32-bit electronic serial number with a 56-bit mobile equipment identifier (often referred to using the acronym MEID: Mobile Equipment IDentifier). Although the need to transition to the 56-bit mobile equipment identifier is widely accepted and incorporated in the IS-2000 standard, no consensus has developed regarding how to modify the aforementioned 42-bit public long code mask to accommodate the proposed 56-bit mobile equipment identifier. One possible technique is to map the 56-bit mobile equipment identifier to a 24-bit value and then to concatenate an 8-bit value to the 24-bit value to form a 32-bit pseudo-ESN. However, this pseudo-ESN method does not result in a sufficient number of distinct public long code masks to prevent an undesirable number of collisions between mobile units.
The public long code mask may alternatively be generated using a base station identifier (sometimes referred to using the acronyms BS_ID or BASE_ID), a Walsh code, and a 9-bit string (sometimes referred to as a PN offset) that differentiates one base station from a neighboring base station. If a call is in handoff, all handoff legs need to use the same public long code mask, i.e. a public long code mask assigned by the primary leg. However, the public long code mask needs to be changed whenever a new primary assumes control of the call, e.g. when a primary transfer occurs, so that the old primary may reuse the Walsh code once the old primary drops off the call. Otherwise, two mobile units on two different calls may be assigned the same public long code mask, which may cause crosstalk between the two concurrent wireless communication links.
During an inter-frequency handoff from a first frequency (F1) to a second frequency (F2) within a sector, the public long code mask generated by a base station typically changes after the inter-frequency hand-off. For example, a mobile unit that has established a first wireless communication link with a first base station (BS1), with a first PN offset (PN_Offset1), on a first carrier frequency (F1) using a first Walsh code (W1) may have a public long code mask of [BS1, PN_Offset1, W1]. The inter-frequency handoff moves the first wireless communication link from the first carrier frequency (F1) to the second carrier frequency (F2) in the same sector (BS1, PN_Offset1). The first base station (B1) assigns the first wireless communication link a second Walsh code (W2). Thus, the new public long code mask may be [BS1, PN_Offset1, W2].
The new public long code mask may be transmitted to the mobile unit either during or after the inter-frequency handoff. If the new public long code mask is transmitted to the mobile unit during the time-critical handoff phase, the additional time delay needed to transmit the public long code mask may increase the number of handoff failures. On the other hand, if the new public long code mask is not transmitted to the mobile unit until after the inter-frequency handoff, the number of collisions may be increased. For example, immediately after the handoff, but before the new public long code mask has been transmitted to the mobile unit, the second carrier frequency (F2) will also be using the public long code mask [BS1, PN_Offset1, W1]. In the meantime, a second wireless communication link that has previously been established in the same sector on the second carrier frequency (F2) may be using the Walsh code (W1). The base station identifier and pilot PN offset should be the same for all carriers in the same sector, so it is very likely that the public long code mask of the second wireless communication link may be [BS1, PN_Offset1, W1]. Consequently, it is very likely that the first wireless communication link will collide with the second wireless communication link during the inter-frequency handoff.
The present invention is directed to addressing the effects of one or more of the problems set forth above.